1. Field of the Invention
The invention concerns a method and a concentrator arrangement adapted to enable a plurality of data terminals to be served by the same B channel of an ISDN (Integrated Services Digital Network) to which the terminals are connected via terminal adapters and a digital subscriber termination to transmit data in packet mode.
2. Description of the Prior Art
Provision is made for connecting data terminals to an access point of a packet-switched network such as the TRANSPAC network via an ISDN to which the terminals are connected via appropriate terminal adapters and via a digital subscriber termination to enable transmission of data in packets, the ISDN having to be connected to the packet-switched network at this access point at least.
It is also feasible to interconnect data terminals via an ISDN digital subscriber termination and possibly via this network for the same purposes of transmitting data in packet mode.
In each case the data is transmitted via an ISDN B channel which has a fixed throughput of 64 kbit/s.
A conventional solution to the problem of connecting an asynchronous data terminal to an ISDN digital subscriber termination uses a terminal adapter through which the terminal is connected at an S reference point of the architectural model defined by the CCITT for this type of network, so-called basic access at this level being usually denoted S0.
The digital subscriber termination, for example a PABX, a local area network or a keyphone installation, is usually connected to an ISDN digital network termination at the T reference point or possibly to a line terminal at a U reference point.
The data terminal may be an asynchronous terminal, for example a personal computer which is connected to the terminal adapter by an RS232C type link under operating conditions equivalent to those recommended by CCITT Recommendation V.24 for transmitting data over telephone lines.
The data terminal may instead be a synchronous terminal, for example a powerful computer system which is connected to the terminal adapter under operating conditions defined by CCITT Recommendation X.25 for packet-switched transmission of data over a public data network.
As a packet mode data terminal usually requires a lower throughput than an ISDN B channel, it is beneficial to concentrate the maximum number of logical channels used by data terminals onto a minimum number of physical channels.
This is particularly important in the case of terminals connected to a packet-switched network via an ISDN access point which inevitably has a limited processing capacity, given the very large number of terminals that can be connected.
This approach is also beneficial when it is required to connect a plurality of data terminals to a common other terminal, for example a computer system which itself has direct access to an ISDN S0 level basic access port.